From fd8a7de177b6f56a0fc59ad211c197a7df06b1ad Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Tue, 20 Jul 2010 14:34:50 +0200 Subject: [PATCH] x86: cpu-hotplug: Prevent softirq wakeup on wrong CPU After a newly plugged CPU sets the cpu_online bit it enables interrupts and goes idle. The cpu which brought up the new cpu waits for the cpu_online bit and when it observes it, it sets the cpu_active bit for this cpu. The cpu_active bit is the relevant one for the scheduler to consider the cpu as a viable target. With forced threaded interrupt handlers which imply forced threaded softirqs we observed the following race: cpu 0 cpu 1 bringup(cpu1); set_cpu_online(smp_processor_id(), true); local_irq_enable(); while (!cpu_online(cpu1)); timer_interrupt() -> wake_up(softirq_thread_cpu1); -> enqueue_on(softirq_thread_cpu1, cpu0); ^^^^ cpu_notify(CPU_ONLINE, cpu1); -> sched_cpu_active(cpu1) -> set_cpu_active((cpu1, true); When an interrupt happens before the cpu_active bit is set by the cpu which brought up the newly onlined cpu, then the scheduler refuses to enqueue the woken thread which is bound to that newly onlined cpu on that newly onlined cpu due to the not yet set cpu_active bit and selects a fallback runqueue. Not really an expected and desirable behaviour. So far this has only been observed with forced hard/softirq threading, but in theory this could happen without forced threaded hard/softirqs as well. It's probably unobservable as it would take a massive interrupt storm on the newly onlined cpu which causes the softirq loop to wake up the softirq thread and an even longer delay of the cpu which waits for the cpu_online bit. Signed-off-by: Thomas Gleixner Reviewed-by: Peter Zijlstra Cc: stable@kernel.org # 2.6.39 --- arch/x86/kernel/smpboot.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c index 33a0c11797de..9fd3137230d4 100644 --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -285,6 +285,19 @@ notrace static void __cpuinit start_secondary(void *unused) per_cpu(cpu_state, smp_processor_id()) = CPU_ONLINE; x86_platform.nmi_init(); + /* + * Wait until the cpu which brought this one up marked it + * online before enabling interrupts. If we don't do that then + * we can end up waking up the softirq thread before this cpu + * reached the active state, which makes the scheduler unhappy + * and schedule the softirq thread on the wrong cpu. This is + * only observable with forced threaded interrupts, but in + * theory it could also happen w/o them. It's just way harder + * to achieve. + */ + while (!cpumask_test_cpu(smp_processor_id(), cpu_active_mask)) + cpu_relax(); + /* enable local interrupts */ local_irq_enable(); -- 2.39.2